Conversion of maleic acid to maleio anhydride



. ra ial July ia isae UNITED STATES PATENT-OFFICE Le Roy U. Spence, Cheltenham, and John 0.

Mitchell, Philadelphia, Pa., assignors-to Biihm 8; Haas Company, Philadelphia, Pa.

No Drawing.

This invention relates to a process of converting maleic acid, into its anhydride.

It is known that maleic acid when heated decomposes into maleic anhydride and water and 5 several processes have .been proposed for distilling out the water in order to prevent its recombining withv the anhydride. To merely heat the acid and thereby vaporizethe'liberated water while preventing the distillation oi the anhydride is 10 not practical for' the'reason that when maleic acid is heated above its melting point it gradually changes to fumaric acid. Because of this, most processes for separating the water vapor have involved methods of reducing the temperature at 15 which the water canbe removed such as the use of vacuuinor the presence of volatile organic hydrocarbons or chlorinated hydrocarbons which on distillation carry over the water vapor. Both these methods have numerous disadvantages and 20 do not without further modification overcome the objectionable formation of fumaric acid.

The vacuum method requires a careful control of temperature and vacuum to prevent vaporization of anhydride andeven when car- 5 ried out under optimum conditions gives a highly corrosive aqueous solution of 'maleic acid as distillate.

- The use of hydrocarbons or chlorinated hydro carbons to assist the removal of watergives a fairly goodyield of anliydride with little iumaric acid formation. This process is usually carried out by distilling out thewater and hydrocarbon or chlorinated hydrocarbon, separating the, water vapor'from the distillate; and returning the solvent to the reaction vessel. Maleic acid is practi-' .cally insoluble in hydrocarbons and chlorinated.

hydrocarbons and. either accumulates on the sides of the reactionvessel or melts and forms a second layer which remains until the reaction is nearly pr-complete. This interferes with the heating of the reaction vessel and tends to cause overheatingof the maleic acid at the sides of the vessel. Molten maleic acid is very corrosive to iron,- alum'inum, stainless steel, etc.,'so that the process is not readily carried out inthe usual equipment Another disadvantage is that readily'available.

- maleicanhydride is also carried into the condenserwith the hydrocarbon or chlorinated hydrocarbon and reacts with the water condensed 50' there, so that the'waterseparated contains. considerable dissolved 'maieic acid.. This causes severe corrosion in the condensers, even when constructed of I aluminum} stainless steel, etc.

the maleic acid" dissolved inthe water fre- 55 quently' represents from. five to ten percent of and reduces the Application October 16, 1937, Serial N0. 169,464

3 Claims. (Cl. 260-341 the charge and must be recovered if good yields are to be realized. when the dehydration is complete, the separation of the anhydride from the solvent requires considerable fractionation because of the tendency of the anhydride to 5 distill over with the solvent.

An efficient and simple method of converting maleic acid into its anhydride is highly desirable for the impurities present inthe crude maleic acid obtained by the catalytic oxidation of benzene, toluene, naphthalene, etc., are more readily removed from the anhydride than from the acid. The uual methods of recrystallization, treatment with decolorizing agents, such as activated carbons, fullers earth, etc., are not efiective and since heating tends to convert the maleic acid to fumaric acid, processes involving a heatingof the acid are not available. 'To obtain a satisfactory product, it is best to convert the.

'crude acid into its anhydride and remove the impurities from the anhydride as, for instance; by the method described in copending applicatlon Serial N0..115,127, illed December 10, 1936.

The object of this invention is to provide a' simple, eflicient and economical methodof con- 2 vertin'g maleic acid into its anhydride, particu larly the crude maleic acid obtained by catalytic oxidation processes. a g

A further object is to provide a method of convertingmaleic acid into its anhydride that can be carried out in apparatus made of the more common structural materials, such as stainless steel, without danger of excessive corrosion.

A'still further object is to provide a method of converting maleic acid into its; anhydride which will give high yields .of the anhyaritie without excessive formation of iumaric acid and without the need of recovering maleic acid from the separatedwater. Other and still further objects will appear from 1 the following description of our process.

These objects are accomplished bythe present invention which is an improvement in the afore- -mentioned process which utilizes. low boiling organic solvents for assisting the removal of water. In general the improvement consistsin the following features: f 1 l.' The use of a high boiling solvent for maleic acid to keep the acid in solution during the dehydration and thereby avoid.local overheating which causes rapid fumaricacid formation. The a dilution of the maleic acid by the solvent also I decreases the rate of fumaric acid formation,

conceive action so that h u process can be carried out in stainless steel without undue corrosion.

2. The use of a low boiling hydrocarbon or chlorinated hydrocarbon to carry out the water slower rate of formation of fumaric acid under these conditions,

3. The use of a fractionating column in the dehydration process. A comparatively short fractionating column will hold back the anhydride when a low boiling compound is used to remove water and permit the separation of the water free from maleic acid. This not only prevents corrosion in the condenser, but also avoids the necessity of recovering maleic acid from'the water of dehydration and thereby improves the yield of maleic anhydride obtained.

Of these steps, numbers 1 and 2 are essential to the process. Number 3 is preferably included but not essential.

When the process is applied to the maleic acid obtained by the catalytic oxidation of hydrocarbons, it is desirable to avoid the absorption of the reaction products in water, since, to recover the crude maleic acid from the water solution, the evaporation must be carried out at a low temperature to avoid the formation of fumaric acid and even when so done the process is complicated by the corrosive action. of the vwater solution of maleic acid. The evaporation may be avoided by contacting the reaction gases with a saturated maleic acid solution so that the product absorbed is converted to solid maleic acid which may be filtered and dried in a current of warm air. Preferably, however, as much of the maleic anhydride as is practical is condensed out of the reaction gases in a condenser operated slightly above the meltingpoint of maleic anhydride to prevent solidification with its resulting plu ng of the condenser and interference with the proper heat transfer. What passes through 'uncondensed may be absorbed in a high-boiling, non-aqueous solvent, preferably a high boiling ester. Since the reaction gases contain considerable water vapor, both the fraction condensed and that absorbed in the high-boiling, non-aqueous liquid will react with the water and give a mixture of acid and a hydride. The relative proportions of acid and anhydride in 'each fraction will vary with the temperature of the condenser and absorber, the concentration of the water vapor in the reaction gases and the time of contact with the reaction gases. However, in the absence of liquid water the corrosive action of the acid at these temperatures is negligible and iron is a suitable material of construction.

As the hishboiling solvent that may be used in the process, compounds that boil higher than maleic anhydride (200 C.) are preferred so that the maleic anhydride can be distilled from the,

solventafter the dehydration is complete. A temperature difference of 30 C. or more is desirable for easy separation. The required solvent capacity of the highboiling solvent will depend on the nature of the material being treated. If

' it is substantially all maleic acid, the solvent med must be capable of holding the entire quantity in solution at-the temperature of dehydration. High boiling esters, such as dimethyl and diethyl phthalatc, dihutyl maleate, ethyl laurate, butyl The lower temperatures are preferred because of the stearate are especially suited for this purpose.

If a mixture of maleic acid and its anhydride'is to be dehydrated, other solvents which are poorer solvents for maleic acid may be used due to the solvent action of'the anhydride present. If desired, maleic anhydride may be added to maleic acid in order that these poorer solvents might be used or maleic anhydride can itself be used as the solvent. Among the liquids that may be used for mixtures of maleic acid and anhydride are such solvents as 'a-chloronaphthalene, dipheriyl, diphenyl ether, a-methyl naphthalene, a-naphthyl methyl ether and Z-nitro-cymene. The following list gives the melting point and boiling point of some suitable solvents.

Compound g g Boiling point Dcqrccs C.

Maleic anhydride Amyl phtha1ate.. 204(11 mm) Anethole 2 n Butyl male'ite 128 (4 mm n Butyl oleate 190 (2 mm n Butyl stearate.

n Butyl oxalate.. 243

n-Butyl benzoate. 249

n-Butyl phthalate 338 a. Chloronaphthalene 263 Diethyl phthala -l 295 Dimethyl phthalate 282 Diphen 4 5 Di pheny] etherl 259 Diethyl malesie 225 a-Methyl naphthalene. 240

fi-Methyl naphthalene. 32 242 a-Naphthyl methyl ether 265 Phthulic anhydride 131 285 As low boiling hydrocarbon or chlorinated hydrocarbon, compounds boiling below 130 C. are

preferred so that they may be easily separated after dehydration is complete. Among the low boiling compounds suitable for carrying over the water vapor are:

Of these,benzene is the most convenient to use when engaged in the manufacture of maleic acid from benzene, because of its availability.

The fractionating column used to hold back the maleic anhydride need only be a relatively short one. Its size and operation will depend upon the low boiling compound used to carry over the water vapor and is easily determinable by those skilled in the art. when benzene is used for this purpose, a column equivalentto from 3 to 5 theoretical plates is suillcient.

Incarryingouttheprocessthemaleicacidor maleic acid-anhydride mixture is the high boiling solvent, or the product frpma scrubbing system using, one of these solvents to recover the'products of a catalytic process is taken, and suflicient low boiling oompoundis added so that the mixture boilsat between 110-160 C. at atmospheric pressure. The

wateristhen distilledoi! withthe aid'ofthelow action vessel and thus keepsthe maleic acidout of the water removed. The low boiling compound is separated from-the water carried over and is'returned as reflux for the column.

. It is preferable to "start the dehydration at 110-130'C. and then,- when the water is nearly all removed, some oi the low boiling solvent is distilled ofi to raise the reaction temperature to 150-170 C. in order that the dehydration may be completed in a short time. Thus, by starting at a low temperature, the fumaric acid formation is kept down, andwhen nearly all-the water has been removed the dehydration rate can be increased and brought to completion in a short time.

The invention is illustrated by the following examples:

Example 1.58 g. of maleic acid, 47 g. of. maleic anhydride, and 37 cc. of benzene were placed in a 'flask fltted with a short iractionating column equivalent to three theoretical plates. The top of the iractionating column was connected to a reflux condenser and a water-separator which removed water from the reflux liquid before it was fed back into the column. The mixture was heated to boiling which started at 112 C. The

water split out of the maleic acid rapidly collected in the trap. The temperature of the re-' action mixture gradually increased. When the water evolution slowed down some of the benzene I was removed, bringing the temperature up to ample 1 and treated in a similar manner. Boiling started at 116 C. and after three hours the temperature was brought up to 144 C. The yield of maleic anhydride was 96.5% of theory, 3.5% being converted to iumaric acid; I

Example 3.A mixture consisting of 232 g. maleic acid, 196 g. maleic anhydride, 250 cc. dimethyl phthalate, and 115 cc. of benzene, was heated for3V2 hours at 128-135" C. in the apparatus of Example 1. The yield of maleic anhydride was-%. This slightly lower yield is due to the longer time of heating andcould beimproved by increasing the temperature near the end of'the reaction.

Example 4.--This example is given toillustrate the-process carried out without the use of a fractionating column. 116 g. of maleic acid, cc. of diethyl phthalate, and 45 cc. of benzene,

. were heated in the appartus used in Exmple 1 from which the fractionating column had been removed, for three-hours at 116-144 C. The water layer removed in the water separator contained 2.7 g. of maleic acid or 2.3% of the charge. The yield of maleic anhydride was 94% of the theoreticaL- We claim: 7 I 1. The process of converting maleic acid to maleic'anhydride which comprises mixing maleic acid with a :highboiling inert solvent for the malei'c acid that boils ata temperature of at.

least 200? C: and a low boiling liquid that is a member oi -the group consisting of hydrocarbons and chlorinated hydrocarbons that b'oil below 130 0., heating the mixture under atmospheric pressure and distilling oft the water of reaction "through a ,fractionating column that condenses the maleic anhydride but permits the passage of water vapor and said low boiling compound, separating the said low boiling compound from the water and returning it as refluxfor the fractionatin'g column, continuing the distillation until theevolution of water diminishes, then distilling ofipart of thelow boiling material to raise the acid with a high boiling solvent that is an ester of an organic acid boiling at a tern r'ature of'at least 200 C. and a low boiling liquid that is a member of the group consisting of hydrocarbons and chlorinated hydrocarbons boiling below 130 C,. heating the mixture'under atmospheric pressure and a temperature of from -130 C. and distilling oil the water of reaction through a Iractionating column that condenses the maleic anhydride but permits the passage of water vapor and said low boiling compound, separating the said low boiling compound'from the water and returning it as reflux for the fractionating column, continuing the distillation until the evolution of water diminishes, then distilling of! part of the low boilingmaterial to raise the boiling point of the reaction mixture to 1rom150- 170 C., and distilling out substantially all" the remaining water at said higher temperature.

3; The process of converting maleic acid to maleic anhydride which comprises mixing maleic 'acid with a high boiling inert-solvent boiling at a temperature 01 at least 230 C. and a low boiling liquid that is a member of the group consisting of vhydrocarbons and chlorinated hydrocarbons boiling below C., heating the mixture under atmospheric pressure and a temperature of 110- 130' 0., distilling oil. the water of reaction througha iractionating column that condenses the maleic anhydride but permits the passage .of water vapor and said low boiling compound,

separaIifig said low boiling compound from the water and returning it. as reflux for the fractionating column, continuing the distillation until the evolution of water diminishes, then distilling ofi part of the low boiling material to raise the boiling point oi the reaction mixture to from C., distilling out substantially all the remaining water and the low boilingmaterial and finally, after dehydration is complete, distilling the maleic anhydride from the said high boiling solvent.

LE ROY U.' SPENCE. JOHN C. MITCHELL. 

